Viruses, Viroids and Prions

Beyond the five kingdoms lies a fascinating group of agents that stretch the very definition of what it means to be 'living' -- viruses, viroids and prions.

Viruses. The word 'virus' means venom or poisonous fluid, and viruses are best understood as obligate intracellular parasites -- they can multiply only inside a living host cell and show no signs of life outside one, behaving essentially as inert biochemical particles when isolated. The English bacteriologist Edward Jenner first developed a vaccine against smallpox in 1798 without knowing about the existence of viruses, but it was the Russian botanist D.J. Ivanowsky (1892), studying the mosaic disease of tobacco, who first recognised the causal agent as smaller than bacteria, since it passed through a bacteria-proof filter. A few years later, in 1898, the Dutch scientist M.W. Beijerinck demonstrated that the fluid extracted from a diseased tobacco plant could infect other healthy plants, and he named this infectious agent 'Contagium vivum fluidum' (contagious living fluid), since it multiplied only within a living host cell. The particle was finally crystallised by W.M. Stanley (1935), who showed that viruses could be crystallised like ordinary chemical substances, and this crystalline material was found to be made largely of protein.

Structurally, viruses are acellular organisms that are basically nucleoproteins: a core of genetic material, which may be either RNA or DNA (never both together), is enclosed within a protective protein coat, called the capsid, made up of small subunits known as capsomeres. Because the genetic material of a virus may be either DNA or RNA, viruses are broadly grouped into DNA viruses and RNA viruses. Plant viruses, such as the Tobacco Mosaic Virus (TMV), are generally single-stranded RNA viruses, whereas animal viruses may possess either single-stranded or double-stranded RNA or DNA. Bacteria-infecting viruses, called bacteriophages (or simply phages), are typically double-stranded DNA viruses; a well-studied example is the T-even bacteriophage, with an icosahedral (polyhedral) protein head enclosing its DNA, a collar, a contractile tail sheath, a base plate and tail fibres that attach to a bacterial cell wall before the phage injects its DNA into the host. TMV, on the other hand, has a helical, rod-shaped structure in which a single strand of RNA lies coiled at the centre and is surrounded by a helical arrangement of protein subunits forming the capsid.

Viroids. In 1971, T.O. Diener discovered a new infectious agent that was even smaller than the smallest known viruses and was found to be the cause of potato spindle tuber disease. Diener named this agent a viroid. A viroid differs from a virus in one crucial respect: it lacks the protein coat that always surrounds a virus's genetic material. A viroid consists only of a free RNA molecule of low molecular weight; without a capsid to enclose and protect it, this naked RNA is nevertheless able to infect a host cell and replicate using the host's own cellular machinery. …